Electret transducer cartridge and case

ABSTRACT

An electret microphone is made of a cartridge subassembly including an electret diaphragm fastened to an apertured backplate having diaphragm support means and air vent means for balancing static pressure on both sides of the diaphragm as unitary parts of the plate. This subassembly is held in a housing between a diaphragm contact ring that grounds the metal diaphragm component to the housing and a resilient compression ring that bears on the back plate and the air vent is totally within the housing. The front of the housing has one of several enclosures over the diaphragm for frequency response varying from the voice range to the high fidelity range. A back closure plate, bounding the compression chamber, carries a solid-state preamplifier and a ring-shaped resilient conductor that makes rolling contact with the back-plate electrode of the cartridge.

United States Patent Fraim et al. June 11, 1974 [54] figg gf TRANSDUCER CARTRIDGE FOREIGN PATENTS OR APPLICATIONS 420,390 7/1933 Great Britain 179/111 [75] Inventors: Freeman W. Fraim, Lexington;

Presmn Murphy Weston both of Primary ExaminerKathleen H. Claffy Mass- Assistant Examiner-Thomas L. Kundert [73] Assignee: Thermo Electron Corporation, t Agent, Firm--Ji1me$ Neal Waltham, Mass.

[22 Filed: Apr. 6, 1972 [571 ABSTRACT An electret microphone is made of a cartridge subas- [211 App! 241580 sembly including an electret diaphragm fastened to an apertured backplate having diaphragm support means [52] U.S. Cl...... 179/111 E, 307/88 ET, 179/111 R, and air vent means for balancing static pressure on 179/179 both sides of the diaphragm 'as unitary parts of the [51 Int. Cl l-l04r 19/04 plate. This subassembly is held in a housing between a [58] Field of Search 179/ 111 R, 111 E, 106, diaphragm contact ring that grounds the metal dia- 179/179; 307/88 ET phragm component to the housing and a resilient compression ring that bears on the back plate and the air [56] References Cited vent is totally within the housing. The front of the UNITED STATES PATENTS housing has one of several enclosures over the diaphragm for frequency response varying from the voice 31:15:55 range to the high fidelity range. A back closure plate, 2:5 7: 5 9H95| M l Jr. t u 179" bOUlldlllg the compression chamber, CZllTlCS Z1 SOlld- 2,579,162 12/1951 veneklaSen 179/111 State Preamplifier and a gp resilient conduc- 2,830,240 4/1958 Speer 179/111 tor that makes rolling contact with the back-plate 2,868,894 1/1959 Schultz 179/111 electrode of the cartridge. 3,300,585 [/1967 Reedyk et al.... 179/111 3,373,251 3/1968 Seeler 179/111 11 Clams, 7 Drawmg Figures 3/ /3 W/R 34 /32 m \\\\\\Y /2 2/ /a 22 2/ /9 20 /0 /6 2 ELECTRET TRANSDUCER CARTRIDGE AND CASE BACKGROUND OF THE INVENTION Electrostatic transducers, such as microphones and speakers, have reached the state where, by using electrets, they can dispense with a polarizing voltage supply. Examples are described in US. Pat. No. 3,612,778, entitled Electret Transducer and Method of Making," which is owned by the assignees of this application. With this development comes the possibility to make very small, rugged and lightweight transducers exhibiting the broad-band response that is regarded as high fidelity, as well as narrower response ranges suitable for voice communications. As the attempt is made to fabricate and assemble smaller sizes, however, mechanical and manipulative problems are confronted, and it becomes increasingly difficult to satisfy all the desired specifications of acoustic properties, mechanical and thermal properties, ease of assembly and low cost.

BRIEF SUMMARY OF THE INVENTION In this invention an improved electret cartridge is presented which in.a single subassembly makes provision for desired acoustic, mechanical, thermal and assembly properties. The cartridge itself is straightforward to make. It fits into a housing which can have any one of several designs depending on the intended use and desired response bandwidth. Conveniently, the housing has rectangular outer dimensions, while providing a circular chamber for electro-mechanical components. When the cartridge is fitted into a housing, air leaks from the front to the back of its diaphragm are automatically provided, and the cartridge is held under rim-compression in an operative position, with electrical connections to it reliably made. The housing closure carries on one surface within the housing a solidstate amplifier circuit which is, upon assembly, coupled to the cartridge. The invention is illustrated in a microphone having outer dimensions slightly larger than /2 inch square by Vs inch thick.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged exploded longitudinal sectional view of un-cased components of a transducer according to the invention;

FIG. 2 shows an exploded oblique view of a transducer according to the invention and an electrical schematic circuit as embodied in certain of the parts;

FIG. 3 shows at A, B and C, respectively, a longitudinal section, a bottom and a side view of an assembled transducer;

FIG. 4 shows a modification of the FIG. 3 transducer, in the acoustic portion to the left of dashed line X-X; and

FIG. 5 shows another modification of the acousti portion of the FIG. 3 transducer.

DETAILED DESCRIPTION OF THE DRAWINGS In FIG. 1, the components of an electret cartridge are shown separated from each other to facilitate a clear description of them. The electret 11 is made of a dielectric sheet 12 coated with an electrical conductor 13, here gold, put on for example by vapor-deposition in vacuum to a thickness which typically is between about 01. and 0.01 micron. Electrets suitable for use in the invention, and methods of making them, are described in the aforesaid US Pat. No. 3,612,778. A back plate 15 is made of a single dielectric body 16, which in the present example is a plastics material formed by injection molding, bearing an electrically conductive coat 17, again of gold, for example. The back plate is perforated with several apertures 18 to provide air passages through it; these apertures are preferably formed in the process of molding the dielectric body. A ridge or land 20 extends toward the electret 11 from the peripheral part of the inner surface 21 of the body 16 confronting the surface 19 of the dielectric sheet 12 of the electret. In the case where the cartridge 10 is circular, as in the present example, this ridge 20 is annular in form. If not annular, it may be generally regarded as having a belt-like form. A central projection or post 22 extends from the inner back-plate surface 21 similarly to the ridge 20. The electricallyconductive coat 17 covers the inner surfaces of the apertures 18, the face of the inner back-plate surface 21 not including the projected faces of the ridge 20, and the back or outer surface 24 of the dielectric body 16 nearly but not entirely to the side walls 25.

In practicing the invention, the electret 11 is brought in contact with the back plate 15 and while held under tension to keep it taut the two parts are fastened together to form a dielectric-to-dielectric bond between the ridge 20 and the peripheral portion of the electret dielectric surface 19. Cements can be used, in which case suitable cements are epoxies and acrylates. The dielectric body 16 may be made of a plastics material known as NORYL," a trademark of General Electric Co. for injection molding granules believed to be composed of polyphenylene oxide and polystyrene. Preferably, this body is molded to a tolerance of l mil-inch. Suitable materials for the electret dielectric 12 are mentioned in the above referenced US. Pat. No. 3,612,778. A plastic-to-plastic, or dielectric-todielectric, bond is formed. The center post 22 extends flush into or slightly beyond the plane of the outer surface of the ridge 20, but need not be cemented or otherwise affixed to the electret diaphragm. The electret 12 is thus bonded at its periphery to the outer land or ridge 20 and supported on the center post 22, and is free to move in the recesses annular region of the inner surface 21 of the back plate 15.

A metallized plastic ring 30 confronts the electret conductor 13 in a peripheral region somewhat narrower than the surface of the ridge 20 bonded to the dielectric side 19 of the electret. The body 31 of this ring is made of a plastics material chosen to have thermal properties compatible with those of the back-plate body 16, and it is coated with an electrical conductor 32, e.g: gold, for coupling the electret conductor 13 to a housing (to be described). A gold layer having suitable thickness for use as the electret conductor would be too thin to allow the making of a gold-to-gold bond to the electret. In practice the ring 30 is compressed against the cartridge 10, its compressive force being exerted against the back-plate ridge 20, so that the electret, in addition to being affixed to the back plate, is held in place by compressive force. The aperture 34 in the ring 30 is an acoustic port for the electret.

Two spacers 36 and 37 are provided for use in assembling the electret l0 and conductor ring 30 into a case. The first spacer 36 is a ring which may be made of TEF- LON, trademark of E. I. DuPont de Nemours & So., for a plastic consisting of a tetrafluoroethylene polymer. The second spacer 37 is a ring which may be made of resilient electrically non-conductive rubber, the axial dimension of which is so chosen that in an assembled transducer this spacer will be under compression in the direction of its axis of revolution. For reasons that will presently appear (see FIG. 3) the first spacer ring 36 has the same outer diameter as the electret cartridge and its conductor ring 30, but the second spacer ring 37 has a larger outer diameter. For the same reasons, the back-surface 24 of the back-plate is fitted with one or more radial slots (see FIG. 2) extending to the side walls 25 from the aperture 18. The larger ring 37 makes an air-tight seal against the inner wall of the case 50 to assure that venting occurs substantially only through these slots and forward to the front surface of the diaphragm 11, and not along other paths. This is done so that the frequency response will be predictable as a function of the slot size.

A closure plate 40, made preferably of a rigid ceramic material suitable for carrying printed electrical circuitry, is held against the second spacer 37 in an assembled transducer, with force sufficient to compress the second spacer. As will presently appear, in an application of the invention to a microphone, a preamplifier circuit (FIG. 2) is mounted on the inner surface 41 of the closure plate, and a ring 42 of electrically conductive rubber (e.g: a rubber which is loaded with silver) is affixed to the input terminal of that circuit. The ring is oriented with its axis of revolution parallel to the surface 41 and it is affixed at its periphery to the closure plate. Its diameter is such that when the closure plate is in place in an assembled transducer the ring 42 is brought in contact with the back-plate conductor 17 at the rear surface 24, and is distorted in a direction transverse to its axis of revolution (that is, radially) by compressive force between the back plate and the closure plate. This provides a rolling contact with the backplate conductor 17, which is superior to a wiping or a rigid contact. The closure plate will have several electrical contacts extending from its inner surface 41 to its outer surface 43, for connection to power supply and utilization conductors (not shown). One such contact 44 is shown, and this may be assumed to be a signal output terminal for the preamplifier. When the parts shown in FIG. 1 are assembled together as described above, the holes 18 in the back plate serve as acoustic elements to connect the air volume behind the back plate, bounded by the closure plate 40 and the spacers 36 and 37, to the air volume within the electret cartridge 10. The size of these holes, their position and number, are chosen to obtain a specific frequency response. The above-mentioned slot (or slots) in the rear surface 24 of the back plate acts (or act) as an air leak connecting the air volume behind the back plate with the ambient at the front conductor 13 surface of the electret. The effective size of this air passage is chosen to obtain a specific frequency response.

In FIG. 2, the components of a complete transducer, including a case 50 are shown in exploded view, together with an electrical circuit diagram showing the electrical function of the electret and preamplifier. The case is rectangular and made of metal, having a circular aperture 51 for reception of the conductor ring 30, the electret cartridge 10, and the spacers 36 and 37, in that order. Behind the circular aperture is a flanged square opening 52 for reception of the closure plate which is square in shape. Clearly, the parts in the circular aperture 51 are dimensioned so that when the closure plate 40 is absent the rubber ring 37 extends out of the circular aperture into the square opening 52. The square opening has a back wall 53 and opposed side walls 54, 55. Upstanding on each side wall is a foldable flap 56, 57, respectively, for clamping the closure plate in place as appears in more detail in FIG. 3. The closure plate has three terminal conductors 44, and 46 along one edge, and one side of the square opening 52 is left open, that is, without a side wall, to give ready access to these conductors.

The circuit schematic in FIG. 2 illustrates that in the electret cartridge 10 the dielectric diaphragm member 12 has a permanent electric charge built into it. Electrical contact is made from the diaphragm conductor 13 to ground terminal 45, and from the back plate 15 to the input 61 of a preamplifier 60, having a power supply terminal 46 and the signal output terminal 44. The preamplifier 60 employs a well-known source-follower circuit. The electrical circuit details of the preamplifier circuit are not part of the present invention, and the illustrated circuit is exemplary only. It is realized in the form of an integrated circuit formed and mounted on the inner surface 41 of the closure plate 40. Certain physical or mechanical features of making connections, as to the case and a ground terminal, and connecting to the input of a suitable preamplifier circuit are parts of the present invention, as has appeared in part above. Thus, the conductor 62 connecting the electret cartridge 10 to the input terminal 61 is in practice according to the invention realized with the conductive rubber ring contact member 42, which is permanently bonded to the closure plate 40 and connected to the preamplifier input terminal 61, and which makes a rolling contact with the back-plate conductor 17. The input circuit resistor 65, to be suitable for use with an electret signal source, will be unusually high, for example 2,000 megohms. This is conveniently made by an available thick-film screening technique directly on the inner surface 41 of the closure plate. The contact ring 42 is affixed to one end of this resistor. This combination of a high-resistance input resistor for the preamplifier, with a resilient conductor affixed to the input terminal and making rolling contact under pressure to the cartridge is a novel arrangement that provides low-noise and mechanical stability properties to a microphone made according to the invention, as will be appreciated more fully from the description of FIG. 3 that follows.

The outside surface 43 of the closure plate is metallized and has a coat (not shown) that is electrically conductive, and this coat contacts the casing via the folded flaps 56, 57 (see FIG. 3) to form with the casing an electrostatic shield for the microphone. An insulating film (48, in FIG. 3B) separates the signal and power terminals 44 and 46, respectively, from the metallized coating. A portion of the ground contact 45 engages the conductive metallized coating so that contact between the diaphragm conductor 13 and the ground terminal 45 is thus established through the metallized coating, flaps 56, 57, the housing and the front conductor ring 30. A second alternative conductive path be tween the diaphragm conductor 13 and the ground contact 45 is made by an electrically conductive adhesive (not shown) located between the housing in the square aperture 52 and the part of ground contact 45 which folds under the closure plate to overlie the inner surface 41 (similar to contact 44 shown in FIG. 1).

An assembled transducer, here a microphone embodying the parts and circuit shown in FIG. 2, is shown in FIG. 3. One foldable flap 57 is shown in FIG. 3A folded over the closure plate 40; both flaps 56 and 57 are shown in FIGS. 3B and 3C. FIG. 3A is taken on line A-A in FIG. 3B. The housing 50 has a front wall 58 terminating the circular aperture 51, against which the contact ring 30 bears when the rubber spacer 37 is under compression from the closure plate 40. The front wall is fitted with one or more air-leak slots 71 running from the ambient region in front of the electret conductor 13 to the side wall of the circular aperture 51. The outer diameters of the contact ring 30, cartridge and first spacer ring 36 are all slightly smaller than the inner diameter of the circular aperture 51, within the permissible range of dimensional tolerances, thereby providing an annular air-leak passage 70 from the front airleak slots to the radial slot or slots 72 in the rear surface 24 of the back-plate 15. The locations of two such slots 72 are shown in dotted line in FIGS. 3A and 3B. The continuous air-leak passage thus formed prevents the build-up of differential pressure between the air inside the transducer housing and the ambient.

It will also be seen in FIG. 3A that the conductive contact ring 42 is compressed radially, making a broad rolling-type contact with the back-plate conductor 17 when the transducer is assembled. No friction or scratching contact isinvolved. The fixed contact between the contact ring 42 and the preamplifier input terminal 61 is not strained, either during assembly, or later during use of the transducer. This combination of features reduces the probability of noise arising from a contact, especially in a microphone incorporating an integrated circuit preamplifier as shown for example in FIG. 2. This design, combined with an electret condenser cartridge 10, installed in a suitably rigid housing 50, provides a virtually shock-proof microphone capable of withstanding repeatedly a 6-foot drop on a tiled or hardwood floor without damage or impairment of response.

A typical electret-condenser microphone as shown in FIG. 3 is about A inch square (FIG. 3B) and about half that much thick (FIG. 3A, FIG. 3C). The front cover 58 of the housing 50 can take one of several forms, depending on desired frequency-band response and on the intended use. FIG. 3 shows a configuration suitable for use in a hearing aid. The front cover 58 is provided with a small aperture 76 which has a cross-sectional area that is a minor fraction of the electret diaphragm area, connecting the air volume 75 between the front cover and the electret cartridge 10 with the environment. Frequency response of a microphone using this design will be from a low of about 50 Hz to a high of about 8,000 Hz.

A modification of FIG. 3 to provide a microphone suitable for music use is shown in FIG. 4 where the front cover 58 has a large aperture 77 nearly coextensive with the electret diaphragm area, so that the electret diaphragm is essentially in direct contact with the environment. In this case it is appropriate to provide a protective screen 78 in the front cover. This screen will desirably be electrically conductive, and grounded to the housing 50, so that it can function as an electrostatic shield. Frequency response of this design is between about 50 Hz and 16,000 Hz.

In FIG. 5 the front cover 58 is not perforated or opened at all. A tubular extension 79 at the side of the housing 50 near the front communicates through the housing with the confined air volume confronting the electret diaphragm. This design is useful for telephone headsets in which a speaking tube (not shown) isattached to the tubular extension 79 and extends from a region over the users ear to a region near the users mouth, thus providing an extremely small and light-weight microphone assembly. Frequency response of a microphone using this design is from 50-6,000 I-Iz.

Microphones made as shown in FIG. 3 or FIG. 5 enclose a quantity of air in front of the diaphragm which can change only slowly with changes in outside atmospheric or ambient pressure, access to the front enclosure being severely restricted by the small opening 76 or tube 79. If the back of the cartridge 10 were vented directly to the ambient permitting rapid changes of back pressure with changes in ambient pressure, then a sudden drop in the ambient pressure (as in an explosively depressurized aircraft, for example) would create a sharply-rising differential pressure on the front of the diaphragm which could damage or destroy it. The totally-enclosed air-leak 71-70-72, with no other venting to the atmosphere, prevents the build-up of such a differential pressure, and maintains essentially the same static pressure on both sides of the diaphragm. Thus, for embodiments of the invention like those shown in FIGS. 3 and 5, a simple bleed to the atmosphere is just not good enough.

What is claimed is:

1. An electroacoustic transducer comprising:

a. a cartridge, said cartridge comprising a pair of plates mounted in confronting relation, a first of said plates comprising an electret having a dielectric surface confronting a second of said plates and an opposed metallized surface, and being imperforate and flexible to serve as a diaphragm, said second plate comprising a relatively rigid dielectric body pierced for acoustic airflow to serve as a backplate and having on the surface thereof confronting said dielectric surface a peripheral ridge extending to the peripheral region of said dielectric surface and a supporting element extending from said confronting surface within and spaced from said peripheral ridge to said dielectric surface for making contact with and supporting said first plate in spaced relation from said second plate, a metallic coat on said confronting surface of said body within said ridge and conductor means extending through said body for making electrical contact to said coat from the opposed side of said body, the surface of said ridge confronting said peripheral region of said dielectric surface being free of said coat, and means effecting a dielectric-to-dielectric bond between said surface of said ridge and said peripheral region;

b. a tubular housing having at one end electrically conductive means at least partially closing said one end, said cartridge being disposed in said housing with said metallized surface of said first plate facing to said one end;-

c. an electrically conductive belt-like contact member in electrical contacting relation to said metallized surface of said first plate in a peripheral region thereof opposed to said peripheral ridge of said second plate and bearing against said closing means; d.- a rigid third plate disposed within said housing at the other end to serve as a closure for said housing;

e. an electronic signal transducing circuit formed on the surface of said third plate within said tubular housing, said circuit having a first terminal on said surface, and a second terminal on the opposed surface of said third plate, and resilient electrical conductor means comprising resilientdielectric material loaded with electrically conductive material held under compression between said second and third plates making connection between said first terminal and said electrical contact to said coat on said second plate;

f. resilient belt-like means in said housing under compression between the respective peripheries of said third plate and said opposed side of said body of said second plate; and

g. means holding said third plate fast in said housing,

the space between said first and third plates providing a compression chamber for said diaphragm.

2. An electroacoustic transducer according to claim 1 wherein said electrically conductive belt-like member comprises a a dielectric body coated with an electrical conductor.

3. An electroacoustic transducer comprising:

a. a cartridge, said cartridge comprising a pair of plates mounted in confronting relation, a first of said plates comprising an electret having a dielectric surface confronting a second of said plates and an opposed metallized surface, and being imperforate and flexible to serve as a diaphragm, said second plate comprising a relatively rigid dielectric body pierced for acoustic airflow to serve as a backplate and having on the surface thereof confronting said dielectric surface a peripheral ridge extending to the peripheral region of said dielectric surface and a supporting element extending from said confronting surface within and spaced from said peripheral ridge to said dielectric surface for making contact with and supporting said first plate in spaced relation from said second plate, a metallic coat on .said confronting surface of said body within said ridge and conductor means extending through said body for making electrical contact to said coat from the opposed side of said body, the surface of said ridge confronting said peripheral region of said dielectric surface being free of said coat, and means effecting a dielectric-to-dielectric bond between said surface of said ridge and said peripheral region;

b. a tubular housing having at one end electricallyconductive means at least partially closing said one end, said cartridge being disposed in said housing with said metallized surface of said first plate facing to said one end;

c. an electrically conductive belt-like contact member in electrical contacting relation to said metallized surface of said first plate in a peripheral region thereof opposed to said peripheral ridge of said second plate and bearing against said closing means;

d. a rigid third plate disposed within said housing at the other end to serve as a closure for said housing;

e. an electronic signal transducing circuit formed on the surface of said third plate within said tubular housing, said circuit having a first terminal on said surface, and a second terminal on the opposed surface of said third plate, and resilient electrical conductor means held under compression between said second and third plates making connection between said first terminal and said electrical contact to said coat on said second plate, said resilient conductor comprising an electrically conductive elastomeric ring affixed at its periphery to said first terminal with its axis of revolution substantially parallel to said surface of said third plate within said tubular housing, whereby when placed under compression said ring is distorted in a direction transverse to said axis;

f. resilient belt-like means in said housing under compression between the respective peripheries of said third plate and said opposed side of said body of said second plate; and

g. means holding said third plate fast in said housing,

the space between said first and third plates providing a compression chamber for said diaphragm.

4. An electroacoustic transducer according to claim 3, wherein said electrically conductive belt-like member comprises a dielectric body coated with an electrical conductor.

5. An electroacoustic transducer comprising:

a. a cartridge, said cartridge comprising a pair of plates mounted in confronting relation, a first of said plates comprising an electret having a dielectric surface confronting a second of said plates and an opposed metallized surface, and being imperforate and flexible to serve as a diaphragm, said second plate comprising a relatively rigid dielectric body pierced for acoustic airflow to serve as a backplate and having on the surface thereof confronting said dielectric surface a peripheral ridge extending to the peripheral region of said dielectric surface and along the opposed side thereof away from said diaphragm at least one groove extending to the periphery of said rigid body providing an air leak for permitting air communication from one side of said diaphragm to the other, a metallic coat on said confronting surface of said body within said ridge and conductor means extending through said body for making electrical contact to said coat from the opposed side of said body, the surface of said ridge confronting said peripheral region of said dielectric surface being free of said coat, means effecting a dielectric-to-dielectric bond between said surface of said ridge and said peripheral region, and means to make electrical contact to said metallized surface of said first place;

b. a tubular housing having at one end closure means at least partially closing said one end;

0. means supporting said cartridge in said housing with said diaphragm facing to said closure means and spaced therefrom; and

d. air passage means between the inner-side walls of said housing and the confronting outer peripheral walls of said cartridge communicating with said air leak for permitting air passage from said opposed side of said cartridge to the region between said diaphragm and said closure means.

6. An electroacoustic transducer according to claim in which a transverse dimension of said cartridge is smaller than the corresponding inner transverse dimension of said housing, to provide said air passage means.

7. An electroacoustic transducer according to claim 6 including spacer means between said diaphragm and said closure means for locating said cartridge relative to said closure means, a third rigid plate disposed within said housing at the other end to serve as a back closure plate for said housing, resilient belt-like means in said housing under compression between the respective peripheries of said closure plate and said opposed side of said body of said second plate, said resilient belt-like means forming a seal for establishing a sole fluid communication passage from one side of said diaphragm to the other through said groove, and means holding said closure plate fast in said housing, the space between said first and third plates providing a compression chamber for said diaphragm.

8. An electroacoustic transducer according to claim 5 in which said housing closure means at said one end has an aperture which extends over the major portion of said metallized diaphragm surface.

9. An electroacoustic transducer according to claim 5 in which said housing closure means at said one end overlies substantially all of said metallized diaphragm surface, said closure means having an aperture of which the area is a minor fraction of the area of said metallized surface, thereby defining a substantially fully enclosed volume of air between said housing closure means and said diaphragm.

10. An electroacoustic transducer according to claim 5 having a tubular conduit extending laterally from said housing into the region between said housing closure means at said one end and said metallized diaphragm surface, said closure means fully closing said one end of said housing and covering entirely said metallized diaphragm surface, thereby defining a substantially fully enclosed volume of air between said housing closure means and said diaphragm.

11. An electroacoustic transducer comprising an electrically conductive housing having a front end, a back end, and side walls, a substantially imperforate and flexible electret diaphragm supported in the front end of said housing, electrostatic shield means overlying said diaphragm in said front end, closure means for said back end, said closure means including electrostatic shield means along one surface thereof electrically connected to said housing, power terminal means overlying said shield means, an insulating film covering a portion of said shield means separating said power terminals and said shield means, and ground terminal means engaging said shield means. =l 

1. An electroacoustic transducer comprising: a. a cartridge, said cartridge comprising a pair of plates mounted in confronting relation, a first of said plates comprising an electret having a dielectric surface confronting a second of said plates and an opposed metallized surface, and being imperforate and flexible to serve as a diaphragm, said second plate comprising a relatively rigid dielectric body pierced for acoustic airflow to serve as a backplate and having on the surface thereof confronting said dielectric surface a peripheral ridge extending to the peripheral region of said dielectric surface and a supporting element extending from said confronting surface within and spaced from said peripheral ridge to said dielectric surface for making contact with and supporting said first plate in spaced relation from said second plate, a metallic coat on said confronting surface of said body within said ridge and conductor means extending through said body for making electrical contact to said coat from the opposed side of said body, the surface of said ridge confronting said peripheral region of said dielectric surface being free of said coat, and means effecting a dielectric-todielectric bond between said surface of said ridge and said peripheral region; b. a tubular housing having at one end electrically conductive means at least partially closing said one end, said cartridge being disposed in said housing with said metallized surface of said first plate facing to said one end; c. an electrically conductive belt-like contact member in electrical contacting relation to said metallized surface of said first plate in a peripheral region thereof opposed to said peripheral ridge of said second plate and bearing against said closing means; d. a rigid third plate disposed within said housing at the other end to serve as a closure for said housing; e. an electronic signal transducing circuit formed on the surface of said third plate within said tubular housing, said circuit having a first terminal on said surface, and a second terminal on the opposed surface of said third plate, and resilient electrical conductor means comprising resilient dielectric material loaded with electrically conductive material held under compression between said second and third plates making connection between said first terminal and said electrical contact to said coat on said second plate; f. resilient belt-like means in said housing under compression between the respective peripheries of said third plate and said opposed side of said body of said second plate; and g. means holding said third plate fast in said housing, the space between said first and third plates providing a compression chamber for said diaphragm.
 2. An electroacoustic transducer according to claim 1 wherein said electrically conductive belt-like member comprises a a dielectric body coated with an electrical conductor.
 3. An electroacoustic transducer comprising: a. a cartridge, said cartridge comprising a pair of plates mounted in confronting relation, a first of said plates comprising an electret having a dielectric surface confronting a second oF said plates and an opposed metallized surface, and being imperforate and flexible to serve as a diaphragm, said second plate comprising a relatively rigid dielectric body pierced for acoustic airflow to serve as a backplate and having on the surface thereof confronting said dielectric surface a peripheral ridge extending to the peripheral region of said dielectric surface and a supporting element extending from said confronting surface within and spaced from said peripheral ridge to said dielectric surface for making contact with and supporting said first plate in spaced relation from said second plate, a metallic coat on said confronting surface of said body within said ridge and conductor means extending through said body for making electrical contact to said coat from the opposed side of said body, the surface of said ridge confronting said peripheral region of said dielectric surface being free of said coat, and means effecting a dielectric-to-dielectric bond between said surface of said ridge and said peripheral region; b. a tubular housing having at one end electrically-conductive means at least partially closing said one end, said cartridge being disposed in said housing with said metallized surface of said first plate facing to said one end; c. an electrically conductive belt-like contact member in electrical contacting relation to said metallized surface of said first plate in a peripheral region thereof opposed to said peripheral ridge of said second plate and bearing against said closing means; d. a rigid third plate disposed within said housing at the other end to serve as a closure for said housing; e. an electronic signal transducing circuit formed on the surface of said third plate within said tubular housing, said circuit having a first terminal on said surface, and a second terminal on the opposed surface of said third plate, and resilient electrical conductor means held under compression between said second and third plates making connection between said first terminal and said electrical contact to said coat on said second plate, said resilient conductor comprising an electrically conductive elastomeric ring affixed at its periphery to said first terminal with its axis of revolution substantially parallel to said surface of said third plate within said tubular housing, whereby when placed under compression said ring is distorted in a direction transverse to said axis; f. resilient belt-like means in said housing under compression between the respective peripheries of said third plate and said opposed side of said body of said second plate; and g. means holding said third plate fast in said housing, the space between said first and third plates providing a compression chamber for said diaphragm.
 4. An electroacoustic transducer according to claim 3, wherein said electrically conductive belt-like member comprises a dielectric body coated with an electrical conductor.
 5. An electroacoustic transducer comprising: a. a cartridge, said cartridge comprising a pair of plates mounted in confronting relation, a first of said plates comprising an electret having a dielectric surface confronting a second of said plates and an opposed metallized surface, and being imperforate and flexible to serve as a diaphragm, said second plate comprising a relatively rigid dielectric body pierced for acoustic airflow to serve as a backplate and having on the surface thereof confronting said dielectric surface a peripheral ridge extending to the peripheral region of said dielectric surface and along the opposed side thereof away from said diaphragm at least one groove extending to the periphery of said rigid body providing an air leak for permitting air communication from one side of said diaphragm to the other, a metallic coat on said confronting surface of said body within said ridge and conductor means extending through said body for making electrical contact to said coat from the opposed side of said body, the surface of said ridge confRonting said peripheral region of said dielectric surface being free of said coat, means effecting a dielectric-to-dielectric bond between said surface of said ridge and said peripheral region, and means to make electrical contact to said metallized surface of said first place; b. a tubular housing having at one end closure means at least partially closing said one end; c. means supporting said cartridge in said housing with said diaphragm facing to said closure means and spaced therefrom; and d. air passage means between the inner-side walls of said housing and the confronting outer peripheral walls of said cartridge communicating with said air leak for permitting air passage from said opposed side of said cartridge to the region between said diaphragm and said closure means.
 6. An electroacoustic transducer according to claim 5 in which a transverse dimension of said cartridge is smaller than the corresponding inner transverse dimension of said housing, to provide said air passage means.
 7. An electroacoustic transducer according to claim 6 including spacer means between said diaphragm and said closure means for locating said cartridge relative to said closure means, a third rigid plate disposed within said housing at the other end to serve as a back closure plate for said housing, resilient belt-like means in said housing under compression between the respective peripheries of said closure plate and said opposed side of said body of said second plate, said resilient belt-like means forming a seal for establishing a sole fluid communication passage from one side of said diaphragm to the other through said groove, and means holding said closure plate fast in said housing, the space between said first and third plates providing a compression chamber for said diaphragm.
 8. An electroacoustic transducer according to claim 5 in which said housing closure means at said one end has an aperture which extends over the major portion of said metallized diaphragm surface.
 9. An electroacoustic transducer according to claim 5 in which said housing closure means at said one end overlies substantially all of said metallized diaphragm surface, said closure means having an aperture of which the area is a minor fraction of the area of said metallized surface, thereby defining a substantially fully enclosed volume of air between said housing closure means and said diaphragm.
 10. An electroacoustic transducer according to claim 5 having a tubular conduit extending laterally from said housing into the region between said housing closure means at said one end and said metallized diaphragm surface, said closure means fully closing said one end of said housing and covering entirely said metallized diaphragm surface, thereby defining a substantially fully enclosed volume of air between said housing closure means and said diaphragm.
 11. An electroacoustic transducer comprising an electrically conductive housing having a front end, a back end, and side walls, a substantially imperforate and flexible electret diaphragm supported in the front end of said housing, electrostatic shield means overlying said diaphragm in said front end, closure means for said back end, said closure means including electrostatic shield means along one surface thereof electrically connected to said housing, power terminal means overlying said shield means, an insulating film covering a portion of said shield means separating said power terminals and said shield means, and ground terminal means engaging said shield means. 